<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta name="generator" content="rustdoc">
<meta name="description" content="API documentation for the Rust `FnOnce` trait in crate `std`.">
<meta name="keywords" content="rust, rustlang, rust-lang, FnOnce">
<title>std::ops::FnOnce - Rust</title>
<link rel="stylesheet" type="text/css" href="../../normalize.css">
<link rel="stylesheet" type="text/css" href="../../rustdoc.css">
<link rel="stylesheet" type="text/css" href="../../main.css">
<link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico">
</head>
<body class="rustdoc trait">
<!--[if lte IE 8]>
<div class="warning">
This old browser is unsupported and will most likely display funky
things.
</div>
<![endif]-->
<nav class="sidebar">
<a href='../../std/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a>
<p class='location'>Trait FnOnce</p><div class="block items"><ul><li><a href="#associated-types">Associated Types</a></li><li><a href="#required-methods">Required Methods</a></li><li><a href="#implementors">Implementors</a></li></ul></div><p class='location'><a href='../index.html'>std</a>::<wbr><a href='index.html'>ops</a></p><script>window.sidebarCurrent = {name: 'FnOnce', ty: 'trait', relpath: ''};</script><script defer src="sidebar-items.js"></script>
</nav>
<nav class="sub">
<form class="search-form js-only">
<div class="search-container">
<input class="search-input" name="search"
autocomplete="off"
placeholder="Click or press ‘S’ to search, ‘?’ for more options…"
type="search">
</div>
</form>
</nav>
<section id='main' class="content">
<h1 class='fqn'><span class='in-band'>Trait <a href='../index.html'>std</a>::<wbr><a href='index.html'>ops</a>::<wbr><a class="trait" href=''>FnOnce</a></span><span class='out-of-band'><span class='since' title='Stable since Rust version 1.0.0'>1.0.0</span><span id='render-detail'>
<a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">
[<span class='inner'>−</span>]
</a>
</span><a class='srclink' href='../../src/core/ops/function.rs.html#216-224' title='goto source code'>[src]</a></span></h1>
<pre class='rust trait'><div class="docblock attributes">#[lang = "fn_once"]
</div>pub trait FnOnce<Args> {
type <a href='#associatedtype.Output' class="type">Output</a>;
extern "rust-call" fn <a href='#tymethod.call_once' class='fnname'>call_once</a>(self, args: Args) -> Self::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a>;
}</pre><div class='docblock'><p>The version of the call operator that takes a by-value receiver.</p>
<p>Instances of <code>FnOnce</code> can be called, but might not be callable multiple
times. Because of this, if the only thing known about a type is that it
implements <code>FnOnce</code>, it can only be called once.</p>
<p><code>FnOnce</code> is implemented automatically by closure that might consume captured
variables, as well as all types that implement <a href="trait.FnMut.html"><code>FnMut</code></a>, e.g. (safe)
<a href="../../std/primitive.fn.html">function pointers</a> (since <code>FnOnce</code> is a supertrait of <a href="trait.FnMut.html"><code>FnMut</code></a>).</p>
<p>Since both <a href="trait.Fn.html"><code>Fn</code></a> and <a href="trait.FnMut.html"><code>FnMut</code></a> are subtraits of <code>FnOnce</code>, any instance of
<a href="trait.Fn.html"><code>Fn</code></a> or <a href="trait.FnMut.html"><code>FnMut</code></a> can be used where a <code>FnOnce</code> is expected.</p>
<p>Use <code>FnOnce</code> as a bound when you want to accept a parameter of function-like
type and only need to call it once. If you need to call the parameter
repeatedly, use <a href="trait.FnMut.html"><code>FnMut</code></a> as a bound; if you also need it to not mutate
state, use <a href="trait.Fn.html"><code>Fn</code></a>.</p>
<p>See the <a href="../../book/second-edition/ch13-01-closures.html">chapter on closures in <em>The Rust Programming Language</em></a> for
some more information on this topic.</p>
<p>Also of note is the special syntax for <code>Fn</code> traits (e.g.
<code>Fn(usize, bool) -> usize</code>). Those interested in the technical details of
this can refer to <a href="../../nomicon/hrtb.html">the relevant section in the <em>Rustonomicon</em></a>.</p>
<h1 id='examples' class='section-header'><a href='#examples'>Examples</a></h1>
<h2 id='calling-a-by-value-closure' class='section-header'><a href='#calling-a-by-value-closure'>Calling a by-value closure</a></h2>
<pre class="rust rust-example-rendered">
<span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="number">5</span>;
<span class="kw">let</span> <span class="ident">square_x</span> <span class="op">=</span> <span class="kw">move</span> <span class="op">||</span> <span class="ident">x</span> <span class="op">*</span> <span class="ident">x</span>;
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">square_x</span>(), <span class="number">25</span>);<a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%20main()%20%7B%0Alet%20x%20%3D%205%3B%0Alet%20square_x%20%3D%20move%20%7C%7C%20x%20*%20x%3B%0Aassert_eq!(square_x()%2C%2025)%3B%0A%7D">Run</a></pre>
<h2 id='using-a-fnonce-parameter' class='section-header'><a href='#using-a-fnonce-parameter'>Using a <code>FnOnce</code> parameter</a></h2>
<pre class="rust rust-example-rendered">
<span class="kw">fn</span> <span class="ident">consume_with_relish</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>(<span class="ident">func</span>: <span class="ident">F</span>)
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnOnce</span>() <span class="op">-></span> <span class="ident">String</span>
{
<span class="comment">// `func` consumes its captured variables, so it cannot be run more</span>
<span class="comment">// than once.</span>
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Consumed: {}"</span>, <span class="ident">func</span>());
<span class="macro">println</span><span class="macro">!</span>(<span class="string">"Delicious!"</span>);
<span class="comment">// Attempting to invoke `func()` again will throw a `use of moved</span>
<span class="comment">// value` error for `func`.</span>
}
<span class="kw">let</span> <span class="ident">x</span> <span class="op">=</span> <span class="ident">String</span>::<span class="ident">from</span>(<span class="string">"x"</span>);
<span class="kw">let</span> <span class="ident">consume_and_return_x</span> <span class="op">=</span> <span class="kw">move</span> <span class="op">||</span> <span class="ident">x</span>;
<span class="ident">consume_with_relish</span>(<span class="ident">consume_and_return_x</span>);
<span class="comment">// `consume_and_return_x` can no longer be invoked at this point</span><a class="test-arrow" target="_blank" href="https://play.rust-lang.org/?code=fn%20main()%20%7B%0Afn%20consume_with_relish%3CF%3E(func%3A%20F)%0A%20%20%20%20where%20F%3A%20FnOnce()%20-%3E%20String%0A%7B%0A%20%20%20%20%2F%2F%20%60func%60%20consumes%20its%20captured%20variables%2C%20so%20it%20cannot%20be%20run%20more%0A%20%20%20%20%2F%2F%20than%20once.%0A%20%20%20%20println!(%22Consumed%3A%20%7B%7D%22%2C%20func())%3B%0A%0A%20%20%20%20println!(%22Delicious!%22)%3B%0A%0A%20%20%20%20%2F%2F%20Attempting%20to%20invoke%20%60func()%60%20again%20will%20throw%20a%20%60use%20of%20moved%0A%20%20%20%20%2F%2F%20value%60%20error%20for%20%60func%60.%0A%7D%0A%0Alet%20x%20%3D%20String%3A%3Afrom(%22x%22)%3B%0Alet%20consume_and_return_x%20%3D%20move%20%7C%7C%20x%3B%0Aconsume_with_relish(consume_and_return_x)%3B%0A%0A%2F%2F%20%60consume_and_return_x%60%20can%20no%20longer%20be%20invoked%20at%20this%20point%0A%7D">Run</a></pre>
</div>
<h2 id='associated-types' class='small-section-header'>
Associated Types<a href='#associated-types' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='associatedtype.Output' class='method'><span id='Output.t' class='invisible'><code>type <a href='#associatedtype.Output' class="type">Output</a></code><div class='since' title='Stable since Rust version 1.12.0'>1.12.0</div></span></h3><div class='docblock'><p>The returned type after the call operator is used.</p>
</div></div>
<h2 id='required-methods' class='small-section-header'>
Required Methods<a href='#required-methods' class='anchor'></a>
</h2>
<div class='methods'>
<h3 id='tymethod.call_once' class='method'><span id='call_once.v' class='invisible'><code>extern "rust-call" fn <a href='#tymethod.call_once' class='fnname'>call_once</a>(self, args: Args) -> Self::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a></code></span></h3><div class='stability'><div class='stab unstable'><span class=microscope>🔬</span> This is a nightly-only experimental API. (<code>fn_traits </code><a href="https://github.com/rust-lang/rust/issues/29625">#29625</a>)</div></div><div class='docblock'><p>Performs the call operation.</p>
</div></div>
<h2 id='implementors' class='small-section-header'>
Implementors<a href='#implementors' class='anchor'></a>
</h2>
<ul class='item-list' id='implementors-list'>
<li><code>impl<'a, A, F> FnOnce<A> for <a class="primitive" href="../primitive.reference.html">&'a </a>F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/ops/trait.Fn.html" title="trait std::ops::Fn">Fn</a><A> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = <F as <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a><A>>::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a>;</span></code></li>
<li><code>impl<'a, A, F> FnOnce<A> for <a class="primitive" href="../primitive.reference.html">&'a mut </a>F <span class="where fmt-newline">where<br> F: <a class="trait" href="../../std/ops/trait.FnMut.html" title="trait std::ops::FnMut">FnMut</a><A> + ?<a class="trait" href="../../std/marker/trait.Sized.html" title="trait std::marker::Sized">Sized</a>, </span><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = <F as <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a><A>>::<a class="type" href="../../std/ops/trait.FnOnce.html#associatedtype.Output" title="type std::ops::FnOnce::Output">Output</a>;</span></code></li>
<li><code>impl<'a, A, R> FnOnce<A> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><<a class="trait" href="../../std/boxed/trait.FnBox.html" title="trait std::boxed::FnBox">FnBox</a><A, Output = R> + 'a + <a class="trait" href="../../std/marker/trait.Send.html" title="trait std::marker::Send">Send</a>><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = R;</span></code></li>
<li><code>impl<'a, A, R> FnOnce<A> for <a class="struct" href="../../std/boxed/struct.Box.html" title="struct std::boxed::Box">Box</a><<a class="trait" href="../../std/boxed/trait.FnBox.html" title="trait std::boxed::FnBox">FnBox</a><A, Output = R> + 'a><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = R;</span></code></li>
<li><code>impl<R, F: <a class="trait" href="../../std/ops/trait.FnOnce.html" title="trait std::ops::FnOnce">FnOnce</a>() -> R> FnOnce<<a class="primitive" href="../primitive.tuple.html">()</a>> for <a class="struct" href="../../std/panic/struct.AssertUnwindSafe.html" title="struct std::panic::AssertUnwindSafe">AssertUnwindSafe</a><F><span class="where fmt-newline"> type <a href='#associatedtype.Output' class="type">Output</a> = R;</span></code></li>
</ul><script type="text/javascript" async
src="../../implementors/core/ops/function/trait.FnOnce.js">
</script></section>
<section id='search' class="content hidden"></section>
<section class="footer"></section>
<aside id="help" class="hidden">
<div>
<h1 class="hidden">Help</h1>
<div class="shortcuts">
<h2>Keyboard Shortcuts</h2>
<dl>
<dt>?</dt>
<dd>Show this help dialog</dd>
<dt>S</dt>
<dd>Focus the search field</dd>
<dt>⇤</dt>
<dd>Move up in search results</dd>
<dt>⇥</dt>
<dd>Move down in search results</dd>
<dt>⏎</dt>
<dd>Go to active search result</dd>
<dt>+</dt>
<dd>Collapse/expand all sections</dd>
</dl>
</div>
<div class="infos">
<h2>Search Tricks</h2>
<p>
Prefix searches with a type followed by a colon (e.g.
<code>fn:</code>) to restrict the search to a given type.
</p>
<p>
Accepted types are: <code>fn</code>, <code>mod</code>,
<code>struct</code>, <code>enum</code>,
<code>trait</code>, <code>type</code>, <code>macro</code>,
and <code>const</code>.
</p>
<p>
Search functions by type signature (e.g.
<code>vec -> usize</code> or <code>* -> vec</code>)
</p>
</div>
</div>
</aside>
<script>
window.rootPath = "../../";
window.currentCrate = "std";
</script>
<script src="../../main.js"></script>
<script defer src="../../search-index.js"></script>
</body>
</html>
distrib
>
Mageia
>
6
>
x86_64
>
media
>
core-updates
>
by-pkgid
>
f9731dfa50015223ad228047cd6e9424
>
files
>
5020
